To calculate curtain fabric, multiply your window width by 1.5 to 2.5 (depending on how full you want them), then multiply the result by the drop height from rail to sill or floor. Add extra for hems, seams and pattern repeats. This guide walks you through each measurement and shows you how to account for real-world factors that affect the total yardage you need.
Why measuring twice matters for curtain fabric
Most homeowners underestimate how much fabric a pair of curtains actually needs. The difference between a skimpy, poorly hung pair and curtains that hang well, fold gracefully and last years comes down to accurate measurement and honest calculation at the start. Curtains that are too thin look thin; curtains cut from enough fabric hang with weight and presence, and they wear better because the load is spread across more material.
Before you shop for fabric or contact anyone to make or remake your curtains, you need three hard numbers: the width of the pole or track, the drop distance, and the fullness ratio you want. Each one feeds into the next. If you measure once and guess the rest, you will either buy too little fabric and end up with inadequate curtains, or buy too much and waste money. Taking time to measure properly at home costs nothing and saves both.
What are the three core measurements you need?
The first measurement is the width of your pole or track, measured end to end. Not the window frame – the actual pole or track on which the curtains will hang. This is your starting point. The second is the drop: measure from the bottom of the pole or ring to where you want the curtain to end. Most curtains reach the sill or the floor; some sit just above the radiator. Measure the exact distance. The third is the fullness ratio, which is a multiplier that tells you how much extra fabric you need so the curtains bunch and fold when closed. A ratio of 1.5 means the fabric width (when laid flat and gathered) is 1.5 times the pole width. A ratio of 2 or 2.5 gives a more luxurious, deeply folded look.
Once you have these three numbers, the maths is simple. Multiply the pole width by your chosen fullness ratio to get the total fabric width needed. Then multiply that by the drop distance. For example: a 2-metre pole with a fullness ratio of 2 and a 2.5-metre drop means you need 4 metres of fabric width × 2.5 metres drop = 10 square metres of fabric. But that is not your final answer; you must account for hems, seams and pattern repeats before you buy.
How do hems, seams and pattern repeats change your calculation?
Every curtain needs a hem along the bottom and a side seam on each edge. A standard bottom hem takes 10–15 cm; side seams take 3–5 cm each. If your curtains are lined (common in East London homes where light control and insulation matter), you add a further margin for the lining seams. A pattern repeat – the vertical distance over which a printed or woven pattern repeats – means you must buy extra length so the pattern lines up correctly across both curtains and looks intentional, not shoddy. A large-scale damask or stripe might have a 40–60 cm repeat; a small geometric pattern might be 10–20 cm. Check the fabric swatch or specification before you order.
Add your hem and seam allowances to the drop measurement. If your drop is 2.5 metres and you need 15 cm for the bottom hem plus 5 cm for top fixing, add 0.2 metres: your working drop becomes 2.7 metres. If the fabric has a 50 cm pattern repeat and you are buying 2.7 metres, you may need to round up to the next full repeat (3 metres) to ensure the pattern sits correctly. This is where working with someone experienced matters – a mistake here means fabric that arrives with the pattern misaligned or hems that run out of room.
What does fullness ratio mean, and which one should you choose?
Fullness ratio is not arbitrary; it directly affects how your curtains look when closed and how they hang when open. A ratio of 1.5 is minimal – the curtain barely gathers at all and looks flat and functional. It suits small windows, tight budgets and rooms where you want minimal fabric bulk. A ratio of 2 is standard in most homes and gives a balanced, professional fold. A ratio of 2.5 or higher creates a luxury effect: the fabric bunches deeply when drawn, and the curtains have real visual weight and presence. Pubs and restaurants in Barking and Dagenham often prefer ratios of 2 or higher on booth seating because the heavier fold looks intentional and ages well with wear.
Your choice depends on your budget, your window size and the look you want. Larger windows can carry a higher ratio without looking overwrought; smaller windows benefit from restraint. Heavyweight fabrics (wool, velvet, linen blends) often look best at ratios of 2 or higher because the weight supports the fold. Lightweight fabrics (cotton, linen, synthetics) can work at 1.5–2 without looking skimpy. If you are unsure, 2 is the safe, proven middle ground.
How do you account for fabric width on the bolt?
Curtain fabric comes in standard widths – typically 137 cm (54 inches) or 280 cm (110 inches), though some designer and natural fabrics run different widths. Your calculated fabric width requirement may not be a neat multiple of your fabric bolt width. For example, if you need 4 metres of width and your fabric is 137 cm wide, you cannot buy 4 metres in one piece. You would buy two widths (2 × 137 cm = 2.74 metres) and seam them together down the middle. This is standard practice and invisible when done properly, but it adds to the total length of fabric you need to order.
If your fabric is 280 cm wide and your requirements are less than that, you may be able to cut both curtains from a single width, which saves cost. Conversely, if you are using a patterned fabric and need to match the pattern across seams, you must allow extra length on each width so the pattern aligns. This is why stating the exact bolt width and pattern repeat to your upholsterer or maker matters. A studio in East London working with real projects (like the residential reupholstery work Herd Upholstery handles) will ask for these details before quoting, because they affect the final fabric quantity and cost.
What is the simplest way to get an accurate final figure?
Measure your pole width, drop height and note the fullness ratio and any pattern repeat on your chosen fabric. Write these down. Add 10–15 cm for the bottom hem and 3–5 cm for top and side seams. If the fabric has a pattern repeat, round your drop up to the next full repeat cycle. Then multiply: (pole width × fullness ratio) × (drop + seam allowance). Divide that by the bolt width of your chosen fabric (usually 137 cm or 280 cm). Round up to the next whole number; that is your minimum fabric order in metres.
If you are not confident in the maths, do not guess. A local upholsterer or curtain maker will calculate it for you quickly and accurately once you provide the measurements. Herd Upholstery, for example, works across Barking, Dagenham, Ilford and East London on residential projects and can advise on fabric quantity and suitability as part of a conversation – there is no charge for a consultation or a fixed quote within 48 hours once measurements are confirmed. Buying too little fabric is expensive (you have to reorder and wait); buying too much is wasteful. A proper calculation saves both time and money.